"I can't find my pen.I have to buy it"这句话哪里错了

问题描述:

"I can't find my pen.I have to buy it"这句话哪里错了

I have to buy another one.

it 必须改成one

my pen已经找不到了,后面又说it,buy it丢了还怎么买它,应该为the,same,one.